Caroline Criado-Perez, who said she had got conflicting messages from police about the Twitter threats. Photograph: Chris Ratcliffe/PA

The Metropolitan police have lost evidence relating to death and rape threats made on Twitter against the feminist campaigner Caroline Criado-Perez, she has said.

The writer took to Twitter to vent frustration at the police's apparent loss of information related to the threats. "Great, and now it turns out loads of screencaps I spent hours sending to police seem to have gone missing. FFS," she wrote. Criado-Perez said on Twitter that she was again being bombarded with graphic rape threats by online trolls and said police were "achingly slow" to respond to emails. In a series of tweets she suggested that resending a large number of screencaps would force her to relive the initial trauma she suffered. "Right, now I'm being expected to trawl through screencaps for threats from specific names. Yes that won't be at all traumatising," she wrote, adding: "What I can't face is having to go through the threats all over again to send them to the police. How can they not fucking have them."

In another tweet, she wrote: "I can just about cope with threats. What I can't cope with after that is the victim-blaming, the patronising, and the police record-keeping."

The writer said she had been given "conflicting messages" by the police and told she only had to keep one screencap per abuser to alert them, advice which they had since changed. "Now it seems that unless I have the evidence of all the threats myself, it doesn't count, nothing gets done. No one looked into them," she said, adding: "What also pisses me off is the conflicting info I was given. I was told I only needed to keep one screencap in order to alert the police."

Criado-Perez thanked supporters, but reacted angrily to users on Twitter who told her to "ignore the trolls".

A Scotland Yard spokesman said: "We are urgently trying to contact the complainant so we can discuss with her the issues that she is raising on social network sites, to once again explain our approach and tactics in this investigation to provide both answers for her and clarity. A huge amount of evidence, including hundreds of tweets, has been captured by officers and thoroughly examined. This has been a complex and large investigation that has involved the examination of hundreds of messages sent on Twitter. A number of people have so far been arrested in connection with the complaint, and remain on bail.

"The MPS has worked closely with the CPS to ensure that what we are examining and highlighting for prosecution meets a criminal threshold. Officers have been in ongoing dialogue with the complainant since the investigation started. We are mindful of the truly hurtful and distressing impact that receiving such communication can have, however we must be sure to focus our efforts on the things that we and the CPS believe is a crime and can be prosecuted."